Implantable medical devices (IMDs) for monitoring a physiological condition or delivering a therapy include one or more physiological sensors. Physiological sensors used in conjunction with an IMD provide a signal related to a physiological condition from which a patient state or the need for a therapy can be assessed. Examples of such IMDs include heart monitors, pacemakers, implantable cardioverter defibrillators (ICDs), myostimulators, neurological stimulators, drug delivery devices, insulin pumps, glucose monitors, etc.
Optical sensors are employed in IMDs as physiological sensors configured to detect changes in light modulation by a body fluid or tissue measurement volume due to a change in a physiological condition in the body fluid or tissue. Such optical sensors can be used, for example, for detecting changes in metabolite levels in the blood, such as oxygen saturation levels or glucose level, or changes in tissue perfusion. Monitoring such physiological conditions provides useful diagnostic measures and can be used in managing therapies for treating a medical condition. For example, a decrease in blood oxygen saturation or in tissue perfusion may be associated with insufficient cardiac output or respiratory function. Thus monitoring such signals allows an implantable medical device to respond to a decrease in oxygen saturation or tissue perfusion, for example by delivering electrical stimulation therapies to the heart to restore a normal hemodynamic function. One example of an implantable optical sensor used for monitoring blood oxygen saturation is generally disclosed in commonly assigned U.S. Pat. No. 6,198,952 (Miesel), hereby incorporated herein by reference in its entirety. Cardiac pacemakers that respond to changes in blood oxygen saturation as measured by an optical sensor are generally disclosed in U.S. Pat. No. 4,202,339 (Wirtzfeld) and in U.S. Pat. No. 4,467,807 (Bornzin), both of which patents are incorporated herein by reference in their entirety.
Implantable optical sensors typically include a set of light emitting diodes (LEDs), for example two or more LEDs, each emitting a different narrow band of light, and a photodetector for detecting emitted light that is scattered by body fluid or tissue back to the sensor. A current signal emitted by the photodetector in response to the scattered light incident on the photodetector is correlated to a physiological change in the adjacent body fluid or tissue. It is generally desirable to provide sensitive light detection to achieve reliable measurements and to minimize the size of such implantable sensors to minimize patient discomfort and to facilitate the ease of implantation procedures.